This invention relates to fluid seals, and more particularly to a fluid seal for a fluid sterilizing filter cartridge.
Fluids such as water and other chemicals can be purified by passing the fluids through a filter material. The filter material may be adapted to remove particulate matter from the fluids. In addition, the filter material may also be designed to bind impurities such as ions or minerals dissolved in the fluids. Typically, such filter materials are contained in filter cartridges. The fluid that is to be filtered and/or purified may pass from one end of the cartridge to the other, thereby passing through the filter material contained in the cartridge. Alternatively, the filter cartridge may have a permeable cylindrical housing and the fluid that is to be filtered and/or purified passes through the housing and then the filter material into a inner core, upon which the filtered fluid exits the filter cartridge through an opening in one of the ends of its housing.
Certain industries, such as the photo resist industry, require filters for ultra purification of fluids. Such filters must remove not only particulate matter but also ions (especially metal ions) and minerals as described above. Filters used in ultra purification should not contribute impurities to the fluid that is to be purified. The components of such filters should not be made of materials that contain "extractables," which are chemicals that leach into the fluid that is to be purified. Accordingly, the housing and other components of a filter cartridge for ultra purification are typically constructed of inert thermoplastics such as polytetrafluroethylene (PTFE), commercially known as Teflon.RTM., or polypropylene, both of which are materials that are very low in extractables yet have an appropriate toughness, strength, and temperature resistance to be useful in forming the components of filter cartridges.
Filter cartridges usually are designed to mate or attach at one end to a drain for filtered fluid or a source of unfiltered fluid. "Unfiltered" refers to fluid that has yet to pass through the filter material in the filter cartridge, at least on the current pass. In the case of flow-through filter cartridges, the end of the filter cartridge through which unfiltered fluid enters may be mated to a receptacle for providing the unfiltered fluid to the cartridge or may be open within a housing that supplies the unfiltered fluid to the cartridge. The other end of the flow-through filter cartridge may be mated to a receptacle for receiving filtered fluid from the filter cartridge. Filter cartridges with housings that are permeable to unfiltered fluid are usually mated to receptacles that receive filtered fluid from the cartridges.
Means for sealing the juncture between that portion of a filter cartridge that is attached to a receptacle and the receptacle itself must be provided in order to avoid contamination of filtered fluid with unfiltered fluid or to prevent leaks of fluid from the filtration system.
It is known to use a knife edge, typically in the form of a ring extending inwardly from a metallic housing around an inlet or an outlet against which an end of a filter cartridge is compressed. That end of the filter cartridge is typically formed of an elastomeric substance and the knife edge indents the compressible material of the end of the filter cartridge in order to provide a fluid tight seal. The disadvantage of this method of sealing the juncture between the receptacle and the filter cartridge is that the filter cartridge must be kept axially compressed tightly against the housing. This method of sealing the filter cartridge does not provide much tolerance for movement or changes in positioning of the filter cartridge.
It is also known to use an axially compressible seal for a filter cartridge, such as that described in U.S. Pat. No. 5,399,264 to Pulek et al. The successful employment of this seal also requires that the filter cartridge be tightly compressed against a housing.
O-rings are the most common means of sealing a filter cartridge used for sterilizing fluids. Typically, an end of the filter cartridge is tubular and is intended to fit within a cup-like receptacle that receives or provides a flow of fluid from or to the filter cartridge. Circumferential grooves in the walls of the tubular end receive O-rings (two is the standard) that compress radially inwardly when the tubular end is inserted into the cup-like receptacle. The use of O-rings provides a very good seal that accommodates the normal tolerances of construction of the receptacle and the tubular end as well as variations in the positioning of the filter cartridge within the receptacle.
Because O-rings must be compressible to perform a sealing function, they are typically constructed of elastomeric materials that have extractables and may be degraded by the chemicals that are to be purified. Examples of such elastomeric materials are EPR, silicone, Viton and Fiuna. In order to reduce the contribution of extractables to the filtered fluid, manufacturers have encased O-rings in PTFE, thus sealing in the extractables contained in the material of the O-ring while retaining the resilience of the underlying elastomeric material. O-rings constructed in this manner, however, are expensive. The O-rings can contribute extractables to the filtered fluid if their PTFE coating cracks or breaks.
Accordingly, industries that must use ultrapure water and chemicals need a filter cartridge with means for a good seal that will accommodate construction tolerances as well as variations in position of the filter cartridge with respect to a receptacle, yet contribute no more extractables than would be provided by the inert materials normally used to construct the filter cartridge.